Physicists at Harvard University demonstrated a new cooling technique for …

When you think about the temperatures associated with “cold,” you probably imagine a cold winter day, or a block of ice (32 °F, 0 °C, or 273.15 K). This is downright balmy compared to the nanokelvin (10-9 K) temperatures physicists can regularly achieve in the lab. Now, things are about to get even chillier with a new technique that can reduce the entropy—and therefore temperature—of a cold gas to near-absolute zero by finely controlling the number and energy level of atoms.

At near-absolute-zero temperatures, atoms can be held in an optical lattice—formed by standing light waves, where the atoms sit in the troughs of the waves at low potential energy. At these temperatures, they lose most of their thermal fluctuations and begin to act like an ideal quantum system. Atoms held in an optical lattice can be used to simulate electrons trapped in a crystalline solid, so this quantum system can be helpful in studying important phenomena like quantum magnetism and high-temperature superconductivity. The atoms could also be used for quantum logic gates and registers (the working memory of quantum computers).

Unfortunately, to truly create an ideal quantum system, physicists have to reach temperatures extremely close to absolute zero, in the picokelvin (pK, 10-12 K) range. The current record for low temperature is 100 pK, but this wasn’t a gas held in an optical lattice.

Reducing the entropy

In order to reach these ultracold temperatures, a team of Harvard physicists led by professor Marcus Greiner developed a new technique that cools the gas in an optical lattice by reducing the entropy, which is inherently tied to the temperature. According to the third law of thermodynamics, the entropy of a system reaches a minimum at absolute zero, so by reducing the entropy to its lowest achievable value, the temperature can be reduced to just about absolute zero.

One way to think about entropy is as a measure of disorder—the more energetic a system is, the more disordered it is likely to be. In this system, disorder comes from extra atoms sitting in the optical lattice sites. If you can remove the extra atoms, you can make it more orderly and simultaneously reduce the entropy and temperature.

Professor Greiner’s team did this by using a newly discovered phenomenon they call orbital exchange blockade (OEB), which prevents the addition of atoms to a site. An atom occupying an energy level at a single site prevents any further atoms at that site from being excited to the same energy level through repulsive interactions.

This OEB thing sounds great, but how do you exploit it to cool the gas? By changing the intensity of one of the standing light waves that forms the optical lattice, the "depth" of the lattice sites can be modulated. If you match the frequency of this change to the energy gap between the ground and excited levels of the lattice site, you can excite atoms to make the jump.

However, if an atom is already sitting in that excited level, OEB prevents any additional atoms from reaching it, forcing everything else to remain at the ground level. The excited atoms can then be removed from the system, reducing the entropy. Importantly, the excitation frequency depends on the number of atoms at each level, so you can control the final number of atoms at lattice sites.

Demonstrating the blockade

The team demonstrated their technique with two experiments, using a gas of rubidium-87 atoms in a square optical lattice. In the first, they started with a known number of atoms at each site (between one and four) all at the ground energy level. Then, by modulating the frequency, they gradually removed all the extra atoms, finishing with only one in each lattice site—a minimal entropy configuration.

In the second experiment, instead of starting with a known number of atoms all at the ground level, they loaded the lattice with a random number per site, with some excited and some at the ground level. As before, by sweeping the frequency, they removed all the extra atoms.

In the experiments, the team ran into some limitations that prevented optimal cooling and therefore kept the final temperature higher than they'd like. The laser beams used to generate the optical lattice heated the gas slightly, for one thing, although this could be reduced by changing the wavelength used. In addition, inefficiencies in the atom excitation process limited the final entropy.

Even if this technique is never used to reach the picokelvin temperature goal, being able to remove entropy and finely control the number of atoms at specific lattice sites will surely come in handy for quantum computing. In fact, they effectively created the largest quantum register yet, with over 1,000 controllable sites. No wonder, then, that the team leader (Prof. Greiner) won a 2011 MacArthur Foundation “genius” grant for this work.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

No, absolute zero is not attainable*. Think of it this way: in all of the equations describing this, the temperature is in the denominator (i.e. 1/T). If the temperature is ever zero, we get an infinity and the math fails. It's very similar to the speed of light - you can get as close as you want, just don't touch it or everything blows up.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

According to the 3rd Law of Thermal Dynamics, Absolute Zero can never be reached.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

No, absolute zero is not attainable*. Think of it this way: in all of the equations describing this, the temperature is in the denominator (i.e. 1/T). If the temperature is ever zero, we get an infinity and the math fails. It's very similar to the speed of light - you can get as close as you want, just don't touch it or everything blows up.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

According to the 3rd Law of Thermal Dynamics, Absolute Zero can never be reached.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

According to the 3rd Law of Thermal Dynamics, Absolute Zero can never be reached.

Nope. Third law says nothing about that. Heisenberg does.

Actually, one interpretation of the third law (Nernst's postulate) does say: "it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations".

The third law of thermo says something stronger than just "entropy goes to a minimum at absolute zero." It says that the entropy goes to zero. This is a rigorous statement, as far as it goes. Boltzmann showed that entropy is proportional to the log of the multiplicity. The multiplicity is simply a count of the number of different ways a system could have some particular value of the energy. E.g., in a two atom system with three units of energy, the units could be distributed 3 on one atom and none on the other, 3 and 2, 2 and 3 or 0 and 3. That gives a multiplicity of 4. (In technical jargon, multiplicity is the number of microstates corresponding to the same macrostate.)

At absolute zero, you've extracted from a system all the energy it had, leaving it with zero. There is only one way to have no energy at all. The multiplicity of that state is 1 and the log of 1 is zero.

At least, that's the classical point of view. When you add quantum mechanics to the mix, you quickly discover that the uncertainty principle prevents you from extracting all the energy from a system. That would require momentum to be zero with exact precision, no uncertainty, since kinetic energy depends on the square of the momentum. It would also require position to be an exact value, with no uncertainty, since no kinetic energy means no movement. But you can't have both momentum and position uncertainties both zero simultaneously. This is why quantum systems always have non-zero ground state energies, and in turn it is why you can't reach absolute zero.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

According to the 3rd Law of Thermal Dynamics, Absolute Zero can never be reached.

Nope. Third law says nothing about that. Heisenberg does.

Actually, one interpretation of the third law (Nernst's postulate) does say: "it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations".

Sorry, you're right on that, but a postulate does not constitute a proof. Nernst formulated his principle around 1912, prior to the advent of modern quantum theory, so it necessarily had to be a postulate and not a point of fact. It also is not the third law itself, which states that in a perfect crystal when the temperature goes to zero, so does the entropy. Nernst's postulate is an addendum which was required for him to calculate chemical affinities.

I should say also that I rather dislike this characterization of entropy as disorder for a variety of reasons. Entropy is that portion of the energy of a system that is unavailable for doing work. There is a relationship between that and disorder, but it is not one to one. The third law is a good example. When a system is in its ground state, doing work would require reducing its energy below the ground state. Quantum mechanics doesn't allow that. So the ground state energy is pretty much not available for doing work, so the entropy can't ever be zero.

Thank you very much for the neat explanation in you original post. If you don't mind, can you please clarify a little bit about:

pjcamp wrote:

The third law of thermo says something stronger than just "entropy goes to a minimum at absolute zero." It says that the entropy goes to zero. This is a rigorous statement, as far as it goes. Boltzmann showed that entropy is proportional to the log of the multiplicity. The multiplicity is simply a count of the number of different ways a system could have some particular value of the energy. E.g., in a two atom system with three units of energy, the units could be distributed 3 on one atom and none on the other, 3 and 2, 2 and 3 or 0 and 3. That gives a multiplicity of 4. (In technical jargon, multiplicity is the number of microstates corresponding to the same macrostate.)

Would the total number of states = 3^2 = 9 instead of 4? (2 bits, each has 3 states)

It is Thermodynamics 101 that entropy is _not_ a measure of disorder. High entropy equilibrium states can have more microscopic and macroscopic ordered states than low entropy states. (Say, microscopically a crystal vs a gas, macroscopically a liquid + gas phase vs a mixed phase.)

- Macroscopically, entropy is a measure of unavailable heat energy that can't be used to do work. [ See "Thermal physics", Morse, 2nd ed p58.]

- Microscopically, entropy is a measure of available energy states for a system. Those remain to do work, and hence complement the classic definition. [See "Statistical physics", Mandl, p42.]

As a system approaches equilibrium, the system maximizes its spread over energy states, simply because it is the most likely state.

You don't. It is an estimate. http://en.wikipedia.org/wiki/Temperature : "For instance, when scientists at the NIST achieved a record-setting low temperature of 700 nK (1 nK = 10−9 K) in 1994, they used laser equipment to create an optical lattice to adiabatically cool caesium atoms. They then turned off the entrapment lasers and directly measured atom velocities of 7mm per second in order to calculate their temperature."

Never say something is impossible because our knowledge might advance to prove what were laws of physics wrong.

I like to say that thermodynamics is the truest lie you'll ever be told. That means that in a microscopic level, small violations of the 2nd law are possible. However, on a global scale, if your world domination plan collides with thermodynamics, it's not well poised for success.

Have you noticed all the reactions to the FTL neutrino preprint by the Gran Sasso guys? Well, this doesn't happen when someone announces that they have found a large violation of thermodynamics laws. Invariably, those works are *always* wrong. There is a famous quote around by Eddington:

"“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.”

Back on topic, these guys doing extreme cooling always manage to surprise me. Laser cooling was clever, and evaporative cooling was even better. Come on, it worked like blowing on hot coffee. How cool is that?

Semi-off topic: I notice a lot of people say biological systems violate the second (or third law if you consider the "zero law" to be number one) law but that is because they do not consider the surroundings of the system.

entropy must actually be greater than or equal to the system + surroundings

deltaS > Ssys + Ssurr > 0

Entropy should not be thought of as disorder but as energy dispersion; therefore, the second law actually states that energy will always seek better dispersion.

So in biological systems there may seem like there is less "disorder" but there is also always more energy dispersed than used by the system (usually as heat) meaning the second law has not been violated.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

You forget about the law of conservation of energy... Look at a refrigerator: where does the energy in a hot bowl of soup go when you leave it in the freezer overnight? The heat doesn't just disappear, it is absorbed and dissipated by the refrigeration unit. Granted, if you were able to observe something reaching absolute zero, it may appear to cease to exist, but the energy absorbed goes somewhere - the matter now exists as energy.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

You forget about the law of conservation of energy... Look at a refrigerator: where does the energy in a hot bowl of soup go when you leave it in the freezer overnight? The heat doesn't just disappear, it is absorbed and dissipated by the refrigeration unit. Granted, if you were able to observe something reaching absolute zero, it may appear to cease to exist, but the energy absorbed goes somewhere - the matter now exists as energy.

But why would it?

I mean, I get that an atom is densely packed energy, but temperature as we measure it is the movement of that atom. If I stop the atom entirely - align it in perfect order - the energy that makes up the atom can still be bound exactly as it was, it just isn't doing anything.

Isn't the energy that forms the particles in the atom itself distinct from the energy expended for that atom to move? What theory states that densely bound energy quanta (as you would find in a proton) absolutely must move?

And is that really absolute zero? The proton is made up of smaller particles. Maybe they are moving around while the proton is in fact as still as anything ever could possibly detect.

Apparently you've never had to locate the cat five minutes before the vet trip.

That's not "blinking out of existance." See, I always find the cat. The cat is in some impossibly small space where reaching in is nothing but pain. It is transmogrification. From a "cat" into "small ball of teeth and claws 1/8th the size of a regular cat, in an impossible place."

One mistake many people here are making is assuming that, even if atoms behaved in a perfectly classical way, you can stop them. That's not really true. The most efficient cycle possible is a Carnot cycle. This depends on the ratio of initial and final temperatures, and decreases rapidly with T. Thus, in the end we need a collossal amount of energy to remove the last tiny bits of temperature.

This is not unlike the particle accelerators, where we end up using much more energy to accelerate electrons or protons just a few more m/s... without ever getting to c.

@Astlor,

Nothing tells you that a proton must move... but don't expect it to find it anywhere.

I have some questions, someone please enlighten me. Is absolute zero possible? Since energy and mass are interchangeable would chilling something to absolute zero blink it out of existence? Or is there something I am missing?

No, absolute zero is not attainable*. Think of it this way: in all of the equations describing this, the temperature is in the denominator (i.e. 1/T). If the temperature is ever zero, we get an infinity and the math fails. It's very similar to the speed of light - you can get as close as you want, just don't touch it or everything blows up.

I am not trying to be facetious, nor am I stupid enough to think that atoms move in a purely classical way, I am just poking the "what if" button on the quantum thermodynamics textbook...

The thing is, the atoms in the optical lattice are lying in what can be approximated at low temperatures with an harmonic potential. If you removed all temperature, the atoms would lie exactly in the ground state. Such a ground state has a nonzero energy, the famous +1/2\hbar\omega that drove Heisenberg initially crazy, and cannot be called static.

I am not trying to be facetious, nor am I stupid enough to think that atoms move in a purely classical way, I am just poking the "what if" button on the quantum thermodynamics textbook...

The thing is, the atoms in the optical lattice are lying in what can be approximated at low temperatures with an harmonic potential. If you removed all temperature, the atoms would lie exactly in the ground state. Such a ground state has a nonzero energy, the famous +1/2\hbar\omega that drove Heisenberg initially crazy, and cannot be called static.

Thanks for that! More things to learn. For the record, my background in quantum mechanics is "I read some books, and try to pick it up (along with medicine, cosmology, molecular biology and a few other sciences) in my spare time." By day, I'm a sysadmin and a writer.

So if I occasionally run up against some bit of theory that has already been pounded out, it is probably due to not having read enough to truly understand it. I often find myself trapped in these little corridors. “Such and such made so and so go nuts because he couldn’t figure it out” is fairly common. Because it makes me go nuts too!

Unfortunately, it is relatively rare that I run across actual (definitive) answers to most of this. Some hypotheses, few that made it to theory. Lots of speculation and hardened physicists in one camp or another.

I like the “more resources” part, and will take this topic and go learn more. I have always been very interested in the exact properties of bose-einstien condensates, and other properties of matter at or around absolute zero. Truly interesting stuff!

Never say something is impossible because our knowledge might advance to prove what were laws of physics wrong.

There might come a time when it is possible to break the "current" laws of physics, which means "current" laws are wrong, but I wish there came a time when it was possible to actually break the laws of physics, and not cause some sort of universal brainfart and implode reality or something

Kyle Niemeyer / Kyle is a science writer for Ars Technica. He is a postdoctoral scholar at Oregon State University and has a Ph.D. in mechanical engineering from Case Western Reserve University. Kyle's research focuses on combustion modeling.